MAYBE Yuuna know how her lil sis feeling towards her. And she also have a feeling for her lil sis. But she know she cant, and as a bis sis, she need to handle it. Either by dating other people or married someone else to forget the feeling she have for her sister and to save their sibling relationship. And she try anything so that her sister get over it. So when Kiku told yuuna that she want to have her sister, she have a complicated feelings, the times when someone really want her lil sis, will she really able to let go. andd I hope it end up with both siblings get over their incest feelings and move on, Yuuna go with her current lover, and Sena go with Kiku. End story.